1. Field of the Invention
This invention relates to a semiconductor device having TFTs (thin film transistors) provided on an insulating substrate of glass or the like, and a method for producing the semiconductor device.
2. Description of Related Art
TFTs have been conventionally formed on a glass substrate to form a semiconductor device such as an active matrix liquid crystal device or an image sensor. The TFTs are used, for example, to drive the pixels of the liquid crystal device.
The TFTs used in the above devices are generally formed of a silicon semiconductor layer in the form of a thin film. The silicon semiconductor of a thin-film type is classified into two types, an amorphous silicon semiconductor (a-Si) type and a crystalline silicon semiconductor type. The amorphous silicon semiconductor can be relatively easily produced at a low film-forming temperature by a vapor-phase deposition method. Therefore, this type is suitable for mass production, and it has been most generally used. However, this type of silicon semiconductor has inferior physical properties such as electrical conductivity, etc. to the crystalline silicon semiconductor. Therefore, in order to more improve a high-speed response characteristic of TFTs, a producing method for TFTs comprising crystalline silicon semiconductor has been strongly required to be established. As the silicon semiconductor having crystallinity have been known polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between crystallinity and amorphousness, etc.
The following methods may be used to obtain thin film silicon semiconductors having the foregoing crystallinity:
(1) Crystallinity is established during the formation of the semiconductor film.
(2) An amorphous semiconductor film is formed in advance, and then a laser beam is irradiated to the film to crystalize the film.
(3) An amorphous semiconductor film is formed in advance, and then heated to crystalize the film.
However, in the method (1), it is technically difficult to form a film having excellent semiconductor physical properties on the whole surface of a substrate uniformly. In addition, the film formation must be performed at a temperature above 600xc2x0 C. and thus an inexpensive glass substrate is unusable, so that a manufacturing cost is increased.
In the method (2), an excimer laser is most generally used at present as a laser beam source for irradiating a laser beam to an amorphous semiconductor film. In this case, the irradiation area of the laser beam is small, and thus this method has a disadvantage that a throughput is low. In addition, the stability of the laser beam is insufficient, so that the whole surface of a large-area substrate cannot be treated uniformly. That is, this method is not practically usable at present.
As compared to the methods (1) and (2), the method (3) has an advantage that it is more suitable to manufacture a large-area semiconductor film. However, this method requires a heating temperature above 600xc2x0 C., and thus an inexpensive glass substrate is not usable. Therefore, this method must be developed to reduce the heating temperature. Particularly in case of present liquid crystal display devices, a large-area screen design is being promoted, and thus use of a large-size glass substrate is required. When a large-size glass substrate is used, contraction and distortion of a substrate occur in a heating process which is indispensable to produce semiconductors, and they cause a critical problem that the precision of a masking process is reduced. Particularly in a case of 7059 glass which is most generally used at present, it has a distortion point of 593xc2x0 C., and it is greatly deformed in a conventional heat crystallization method. In addition to the heat problem as described above, a heating time required for crystallization is over several tens hours in a present process, and thus the heating time must be shortened.
An object of the present invention is to provide a method capable of solving the above problems, and specifically to provide a process for producing a silicon semiconductor thin film having crystallinity utilizing a method of heating an amorphous silicon thin film to crystallize the thin film, in which both of lowering of the temperature for crystallization and shortage of the heating time for crystallization can be performed. A silicon semiconductor having crystallinity which is manufactured using the process according to this invention has the same physical properties as or physical properties superior to that manufactured by a conventional technique, and it is usable in an active layer area of TFTs.
The inventors of this application have made the following experiments and consideration for a method of forming an amorphous silicon semiconductor film as described above by a CVD method or a sputtering method, and then heating the film to crystalize the film.
An amorphous silicon film is initially formed on a glass substrate, and then the film is crystallized by heating. The inventors investigated the mechanism of this crystallization. Through the experiments, it was observed that crystal growth of silicon starts at an interface between the glass substrate and the amorphous silicon and proceeds vertically to the substrate surface into a pillar shape in the case that the thickness of the film is larger than a certain thickness.
The above phenomenon is considered as progressing on the basis of a mechanism that crystalline nuclei serving as geneses for crystal growth (species serving as geneses for crystal growth) exist between the glass substrate and the amorphous silicon film, and crystal grow from the crystalline nuclei. These crystalline nuclei are considered as being impurity metal elements or crystal components (as is called as a crystallized glass, it is considered that crystal components of silicon oxide exist on the surface of the glass substrate) existing on the surface of the substrate in a very small amount.
Accordingly, it is expected that a crystallization temperature can be lowered by introducing crystal nuclei more positively. In order to confirm an effect of introducing crystal nuclei, the following experiment was tried. That is, a thin film of different metal in a very small amount was beforehand formed on a substrate, then an amorphous silicon thin film was formed on the different metal film, and then heat-crystallization was conducted on the amorphous silicon thin film. As a result, it was proved that the crystallization temperature was lowered when thin films of some different kinds of metal were beforehand formed on the substrate, and it was expected that crystal growth using foreigners as crystal nuclei had conducted. Accordingly, a more detailed mechanism for plural kinds of impurity metal which could lower the crystallization temperature was studied.
The crystallization mechanism can be considered to be classified into two stages which are an initial nucleus generation stage and a subsequent crystal growth stage from the nuclei. The speed of the initial nucleus generation can be detected by measuring a time elapsing until spotted fine crystals occur at a constant temperature. This time could be shortened in all cases where the thin films of the above kinds of impurity metal were formed on the substrate, and the effect of the introduction of the crystal nuclei on the lowering of the crystallization temperature can be proved. As an unexpected result, through an experiment for examining variation of growth of crystal grains with variation of the heating time after generation of crystal nuclei, it was observed that the speed of the crystal growth after the generation of the nuclei was also rapidly increased when a thin film of a certain kind of metal was formed on a substrate, an amorphous silicon thin film was formed on the metal thin film and then the amorphous silicon thin film was crystallized. A mechanism for this effect has not yet been elucidated at present, however, it is guessed that any catalytic effect acts.
At any rate, it was proved that when a thin film was formed of a certain kind metal in a very small amount, an amorphous silicon thin film was formed on the metal thin film and then the amorphous silicon thin film was crystallized by heating, sufficient crystallinity which had not been expected in the prior art could be obtained at a temperature below 580xc2x0 C. and for about 4 hours due to the two effects as described above. Nickel (Ni) is the best material which is experimentally proved as providing the most remarkable effect in all impurity metals having such an effect. In addition to nickel, Fe, Co, Pd and Pt may be listed as a metal element having such a catalytic action on crystallization.
The following is an example showing an effect of formation of a nickel thin film. In a case where an amorphous silicon thin film was formed by a plasma CVD method on a substrate (coring 7059 glass) which had been subjected to no treatment, that is, on which no nickel thin film had been formed, and then heated under a nitrogen atmosphere to crystallize the amorphous silicon thin film, a heating time over ten hours was required for a heating temperature of 600xc2x0 C. On the other hand, in a case where an amorphous silicon thin film was formed on a substrate on which a nickel thin film in a very small amount (hereinafter referred to as a trace nickel thin film) had been formed, the same crystal state as the former case could be obtained by heating the amorphous silicon thin film for about 4 hours. The crystallization of the amorphous silicon thin film was judged using a Raman spectrum in this experiment. From this experiment, it is apparent that nickel has a large effect.
As is apparent from the foregoing, formation of an amorphous silicon thin film after a trace nickel thin film is formed enables the lowering of the crystallization temperature and the shortening of the crystallization time. This process will be described in more detail on the assumption that this process is applied to a TFT producing process. As described later, the same effect can be obtained by forming a nickel thin film on not only a substrate, but also on an amorphous silicon thin film, or by implanting the nickel into the amorphous silicon by an ion implantation method. Accordingly, these treatments are commonly referred to as xe2x80x9ctrace nickel additionxe2x80x9d in the specification of this application.
First, a method for the trace nickel addition will be described.
It has been known that the trace nickel addition can provide the same effect on the lowering of the crystallization temperature in both cases where a trace nickel thin film is formed on a substrate and then an amorphous silicon thin film is formed on the trace nickel thin film, and where an amorphous silicon film is formed and then a trace nickel thin film is formed on the amorphous silicon film, and any film-forming method such as a sputtering method, a deposition method, spin coating, coating or the like may be used as a film-forming method. However, in the method of forming the trace nickel thin film on the substrate, the effect becomes more remarkable by forming a silicon oxide film on a 7059 glass substrate and then forming the trace nickel thin film on the silicon oxide film than by directly forming the trace nickel thin film on the substrate. As one of reasons for this fact, it would be considered that direct contact between silicon and nickel is important for the low-temperature crystallization, and components other than silicon serves to obstruct direct contact or reaction between silicon and nickel in the case of using a 7059 glass substrate.
It was proved that the substantially same effect could be obtained by adding nickel with the ion implantation method as well as the method of forming the trace nickel thin film in contact with the lower surface or upper surface of the amorphous silicon thin film as described above. The lowering of the crystallization temperature was observed for addition of nickel of 1xc3x971015 atoms/cm3 or more. However, it was observed that for addition of nickel of 1xc3x971021 atoms/cm3 or more, the shape of the peak of a Raman spectrogram was clearly different from that of silicon itself, so that a practically usable range of nickel addition is from 1xc3x971015 atoms/cm3 to 5xc3x971019 atoms/cm3. If a nickel concentration is 1xc3x971015 atoms/cm3 or less, nickel elements are localized and thus the catalytic function of nickel is deteriorated. Further, if a nickel concentration is 5xc3x971019 atoms/cm3 or more, nickel and silicon are reacted with each other to form NiSi compounds, and the semiconductor characteristics are hindered. In a crystallized state, products can be more practically used as a semiconductor as the nickel concentration is lower.
On the basis of the above consideration and the fact that products are used as active layers or the like of TFTs, the nickel addition amount is required to be adjusted in the range of 1xc3x971015 atoms/cm3 to 1xc3x971019 atoms/cm3.
Next, crystal form when the trace nickel addition is performed will be described.
As explained below, in the case that nickel is not added, crystallization occurs from crystal nuclei existing at the substrate surface. This crystallization proceeds in a random direction provided that the film is not thicker than a certain thickness. Further, if the thickness of the film is enough large, the columnar crystals grow in such a manner that the (110) direction aligns vertically to the substrate surface. This crystallization can be observed on the entire surface of the substrate. On the other hand, in the case of the trace nickel addition according to this embodiment, different crystal growth was observed between an area added with nickel (hereinafter referred to as xe2x80x9cnickel areaxe2x80x9d) and an area in the neighborhood of the nickel area (hereinafter referred to as xe2x80x9cadjacent areaxe2x80x9d). That is, it became clear from a transmission electron microscopic photograph that, in the nickel area, added nickel or a compound of nickel and silicon served as a crystal nucleus and the pillar-shaped crystal growth progressed substantially vertically to the substrate. In addition, the low-temperature crystallization was also confirmed in the adjacent area in which the nickel was not directly added, and a peculiar crystal growth in which needle or pillar shape crystals grew substantially in parallel to the substrate so that the (111) plane was aligned vertically to the substrate was also observed in this area.
It was observed that the crystal growth in the lateral direction parallel to the substrate was started from the nickel area and the maximum crystal size of grown crystals extended to several hundreds xcexcm, and it became clear that the degree of crystal grow increases with the increase of the time and the temperature. For example, crystal growth of about 40 xcexcm crystals was observed at the temperature of 550xc2x0 C. and for 4 hours. In addition, according to a transmission electron microscopic photograph, each of these large-size laterally-extending crystals was determined to be like monocrystal. Further, the nickel concentration in each of a trace nickel added area, a laterally-extending crystal growth area in the neighborhood of the trace nickel added area and an amorphous area (no low-temperature crystallization appeared in a region extremely distant from the Ni added area) was measured by SIMS (Secondary Ion Mass Spectroscopy). As a result, the nickel concentration in the laterally-extending crystal growth area was measured to be lower than the trace nickel added area by one figure, and diffusion in amorphous silicon was observed. Further, the nickel concentration in the amorphous area was measured to be lower than the laterally-extending crystal growth area by one figure. The relationship between the crystal form and the above result has been unclear at present, however, at any rate, a silicon film having crystallinity of a desired crystal form at a desired area can be formed by adjusting a nickel addition amount and controlling a position where nickel is added.
Next, electrical characteristics of the trace nickel added area and the laterally-extending crystal growth area adjacent thereto will be described.
With respect to conductivity, the trace nickel added area had the substantially same conductivity value as a no-nickel added film, that is, the film which was subjected to crystallization at about 600xc2x0 C. for several tens hours. Further, calculating an activation energy on the basis of temperature-dependence of conductivity, there was observed no behavior which was expected to be induced due to the nickel energy level when the nickel addition amount was set in the range of about 1017 atoms/cm3 to 1018 atoms/cm3. That is, according to only this fact, in a case where the nickel concentration in a crystal silicon semiconductor film is below 1xc3x971018 atoms/cm3, there would occur no problem even if a semiconductor device, for example, a TFT is formed using this film.
On the other hand, the laterally-extending crystal growth area had a higher conductivity than the trace nickel added area by one or more figures, and it has a very higher value as a silicon semiconductor having crystallinity. The reason for this fact would be considered as follows. That is, a current passing direction was coincident with the laterally-growing direction of crystals, and thus there was little or no grain boundary in an electron path between electrodes. This fact is perfectly consistent with the result of the transmission electron microscopic photograph.
However, through more delicate observation of the laterally-growing area of the crystals on the transmission electron microscopic photograph, areas containing crystals growing in a branch form was also observed upon viewing from the upper side of the substrate although the crystallization direction of needle or pillar crystals was parallel to the surface of the substrate. That is, it was observed that the needle or pillar crystals grew in the same direction on the average, however, some crystals grew while being branched in a slant direction.
The inventors have deliberately considered the observation result, and had the following conclusion.
Crystal components of a substrate material existing in a substrate or at an interface portion between the substrate and a semiconductor film, or crystal components in the semiconductor film can serve as nuclei for crystal growth, however, these components obstruct the crystal growth in an uniform direction (unidirection) and promote a random crystal growth in the lateral crystal growth process.
Therefore, this invention is characterized in that crystal components at and in the vicinity of the interface between a substrate area on which the growth is to occur and an amorphous silicon semiconductor film (in this invention, the term xe2x80x9camorphousxe2x80x9d does not mean a perfect amorphous state, but may contain crystal components if the amount of the components is small) are removed to the utmost by an ion implantation of inert elements so that this area is made perfectly amorphous, and then by performing the crystal growth in a lateral direction (a direction parallel to the substrate) in a state where no component serving as a crystal nucleus exists, needle or pillar crystals are grown so that the crystal growing directions thereof are coincident with one another as a whole. Particularly by concentratively implanting inert ions into the substrate, the area in the neighborhood of the surface of the substrate (when a blocking film is formed on the surface of the substrate, the blocking film is regarded as the substrate surface), the interface between the substrate and the semiconductor film and the semiconductor film itself are made perfectly amorphous, so that components having crystallinity which might serve as crystal nuclei are removed as perfectly as possible.
In accordance with another aspect of the invention, it is possible to further improve the characteristics of the thus obtained semiconductor layer by further treating the semiconductor layer with a laser light or an intense light as strong as the laser light. Thereby, components existing at grain boundaries or the like and not having been sufficiently crystallized can be further crystallized. It is assumed that the crystalline components which are produced by the preceding heating step function as nuclei so that the remaining amorphous components can be further crystallized by the photo-annealing.
The invention and its application to the actual semiconductor devices will be more fully understood from the following detailed description, when taken with the appended drawings, in which: